1. Field of the Invention
The present invention relates to an engine control system, in which injection characteristics of individual fuel injectors are learned and an amount of fuel injected from individual fuel injector is controlled based on the injection characteristics.
2. Description of Related Art
Recently, some proposals have been made for improving detection accuracy of an air-fuel ratio in an internal combustion engine. For example, JP-A-2005-207405 proposes the following system: an air-fuel ratio of each cylinder is estimated based on output signals of an air-fuel ratio sensor disposed in an exhaust pipe at a position where exhaust gas streams from plural cylinders merge; an air-fuel ratio deviation among cylinders is calculated; an mount for adjusting the air-fuel ratio of each cylinder is calculated to minimize the air-fuel ratio deviation among cylinders; and the air-fuel ratio of each cylinder is controlled using the calculated amount for adjusting the air-fuel ratio.
On the other hand, JP-A-2-78750 proposes the following system: a target amount of fuel injection for each cylinder and an average amount of fuel injection among all cylinders are calculated based on operating conditions of an engine when the engine is idling; the injection amount for each cylinder is adjusted using a difference between the target amount and the average amount; and thus the average amount of fuel injection for all cylinders converges to the target amount.
As shown in FIG. 3 attached hereto, an injection characteristic (an amount of injected fuel versus time period in which fuel is injected) of an individual injector is not the same as the standard injection characteristic. This means that a certain error (deviation) in the injection amount relative to the standard amount cannot be avoided. The error (deviation) may depend on original individuality of each injector, or it may be caused in a course of actual usage.
Since the deviation from the standard characteristic is unavoidable for each injector, the air-fuel ratio deviation among cylinders cannot be precisely detected in the system disclosed in JP-A-2005-207405. This is because an influence of the injection amount error of each injector is included in the deviation of air-fuel ratio among cylinders. Accordingly, the air-fuel ratio deviation among cylinders due to external disturbances, such as introduction of evaporated gas or a blow-by gas into an intake system, is not accurately detected.
In the system disclosed in JP-A-2-78750, the average amount of fuel injection for all cylinders is converged to a target amount if there is a deviation in the fuel amount injected from each injector in the idling state. However, the error in the injection amount of each injector due to the injection characteristic deviation cannot be adjusted.